Metallocene cationic salts of the general type Cp.sub.2 MR.sup.+ X.sup.- (Cp being a cyclopentadienyl-type ligand; M being a Group IVB metal such as titanium, zirconium, or hafnium, R being alkyl, aryl or hydride; and X being a weakly coordinating anion form the basis for a large family of active, efficient and selective olefin polymerization catalysts. The performance of these systems is exceedingly sensitive to the nature of X.sup.- with X.sup.- being, preferably, RMAQ- (MAO being methylaluminoxane), RB(C.sub.6 F.sub.5).sub.3.sup.-, and B(C.sub.6 F.sub.5).sub.4.sup.-. While B(C.sub.6 F.sub.5).sub.4.sup.- appears to give the most active catalysts, such catalysts suffer from insolubility and thermal instability.
U.S. Pat. No. 5,153,157to G. G. Hlatky et al. discloses catalyst systems in which the anion is said to be bulky, labile and non-coordinateable with the Group IV metallocene containing component. Despite a rather generalized disclosure of possible substitution possibilities on the aromatic hydrocarbon groups of the anion, this patent only exemplifies the use of tetraphenyl borate and tetrapentafluorophenyl borate species without any suggestion that the presence of certain types of substitution on these structures would yield systems having enhanced solubility and thermal stability characteristics. There is, furthermore, no enabling description as to how such enhanced systems might be made.